The present invention relates to a process for producing semiconductors composed of the Group III-V compounds and Group III-V compound semiconductors. More particularly, the present invention relates to a method for growing crystals of Group III-V compound semiconductors used for various semiconductor devices such as laser diodes, light-emitting diodes, high electron-mobility transistors, etc., and Group III-V compound semiconductors obtained by the method.
The following methods have been known for crystal growth of the elements used for the semiconductor devices:
(i) crystal of C.sub.1.sup.III As.sub.x P.sub.1-x (C.sub.1.sup.III : Group III element, 0.ltoreq.x&lt;0.5) is directly grown on A.sub.1.sup.III As.sub.w P.sub.1-w (A.sub.1.sup.III : Group III element with Al composition of less than 0.3, 0.5&lt;w.ltoreq.1) formed on a semiconductor crystal substrate or on an epitaxial film grown on the said substrate; and PA1 (ii) crystal of A.sub.2.sup.III As.sub.u P.sub.1-u (A.sub.2.sup.III : Group III element with In composition of not less than 0.3, 0.ltoreq.u.ltoreq.1) is grown on a semiconductor crystal substrate or on an epitaxial film grown on the said substrate, and after an interruption of crystal growth in which a heating step is required, C.sub.2.sup.III As.sub.z P.sub.1-z crystal (C.sub.2.sup.III : Group III element, 0.ltoreq.z.ltoreq.1) is grown directly on A.sub.2.sup.III As.sub.u P.sub.1-u. PA1 (1) forming a thin crystal film of A.sub.1.sup.III As.sub.w P.sub.1-w (A.sub.1.sup.III represents a Group III element with Al composition of less than 0.3, 0.5&lt;w.ltoreq.1) on a semiconductor crystal substrate or on an epitaxial film grown on the said substrate, PA1 (2) forming a thin film of A.sub.2.sup.III As.sub.u P.sub.1-u (A.sub.2.sup.III represents a Group III element with In composition of not less than 0.3, 0.ltoreq.u.ltoreq.1) on a semiconductor crystal substrate or on an epitaxial film grown on the said substrate, PA1 (1) a thin crystal film of A.sub.1.sup.III As.sub.w P.sub.1-w (A.sub.1.sup.III represents a Group III element with Al composition of less than 0.3, 0.5&lt;w.ltoreq.1) formed on a semiconductor crystal substrate or on an epitaxial film grown on the said substrate, PA1 (2) a thin film of A.sub.2.sup.III As.sub.u P.sub.1-u (A.sub.2.sup.III represents a Group III element with In composition of not less than 0.3, 0.ltoreq.u .ltoreq.1) formed on a semiconductor crystal substrate or on an epitaxial film grown on the said substrate,
For crystal growth of (Al.sub.y Ga.sub.1-y).sub.0.5 In.sub.0.5 P (0.ltoreq.y.ltoreq.1) which is a material of visible laser diodes or visible light emitting diodes, generally a GaAs buffer layer is grown on a GaAs substrate and the said crystal material is directly grown on the GaAs buffer layer by the MOVPE (metal organic vapor phase epitaxy).
However, the direct growth of (Al.sub.y Ga.sub.1-y).sub.0.5 In.sub.0.5 P (0.ltoreq.y.ltoreq.1) crystal on the GaAs buffer layer formed on a GaAs substrate involves problems that substitution takes place between As in the GaAs crystal and P in the vapor phase to cause interfacial defects, thereby making it unable to obtain AlGaInP crystal with good quality. Especially in growth of AlGaInP crystal with high Al composition, the crystal growth needs to be carried out at a high temperature (not less than 700.degree. C.) for preventing contamination of the crystal with oxygen, so that the influence of the interfacial defects due to substitution between As in the GaAs crystal and P in the vapor phase was vital.
Also, growth of the GaAs buffer layer on a semiconductor substrate is preferably carried out at a low temperature of less than 700.degree. C. for minimizing diffusion of impurities from the substrate crystal into the epitaxial crystal layer. Further, in case the buffer layer has a high In content as A.sub.2.sup.III As.sub.u P.sub.1-u (A.sub.2.sup.III : Group III element with In composition of not less than 0.3, 0.ltoreq.u.ltoreq.1), when the crystal growth is carried out at a high temperature, it is required to supply a large quantity of In since the In incorporation efficiency into the crystal is reduced. Thus, it is advantageous to carry out the crystal growth of the A.sub.2.sup.III As.sub.u P.sub.1-u buffer layer (A.sub.2.sup.III : Group III element with In composition of not less than 0.3, 0.ltoreq.u.ltoreq.1) at a low temperature (less than 700.degree. C.).
On the other hand, the light-emitting layer of a light-emitting semiconductor device needs to have a high luminous efficiency as well as good crystal quality. Therefore, it is generally preferable that the growth of the light-emitting layer be carried out at a high temperature. Especially in the case where the light-emitting layer contains Al (for example, layer composed of AlGaInP or AlGaAs), when the crystal growth is carried out at a low temperature (less than 700.degree. C.), Al element which is one of components of the layer is reacted with oxygen remaining in a trace amount in the growth gas to cause contamination of the crystal with the oxides or occurrence of micro-defects, so that it is impossible to obtain a crystal with high luminous efficiency and good quality. Therefore, it is preferable that the growth of AlGaInP or AlGaAs be carried out at a high temperature (not less than 700.degree. C.) so as to prevent the grown crystal from being contaminated with the Al and oxygen reaction product.
For these reasons, in the crystal growth method in which A.sub.2.sup.III As.sub.u P.sub.1-u crystal is first grown on a substrate and then AlGaInP or AlGaAs crystal is grown on the A.sub.2.sup.III As.sub.u P.sub.1-u buffer layer, it is usual practice to carry out the growth of A.sub.2.sup.III As.sub.u P.sub.1-u crystal (A.sub.2.sup.III : Group III element with In composition of not less than 0.3, 0.ltoreq.u.ltoreq.1) at a low temperature (around 660.degree. C.) and then elevate the temperature to conduct the growth of AlGaInP or AlGaAs crystal at a high temperature of not less than 700.degree. C.
According to this method, however, a part of In in the A.sub.2.sup.III As.sub.u P.sub.1-u crystal grown at a temperature is desorbed in the course of heating due to high vapor pressure, thereby causing surface defects of the grown crystal. The AlGaInP or AlGaAs crystal grown thereon inherits these defects, so that it is impossible to obtain the desired high-quality AlGaInP or AlGaAs crystal.
As a result of intensive studies for overcoming these technical problems, it has been found that by growing a thin crystal film of B.sup.III As (B.sup.III represents a Group III element with Al composition of not less than 0.3) as a protective film on a thin film of A.sub.1.sup.III As.sub.w P.sub.1-w (A.sub.1.sup.III represents a Group III element with Al composition of less than 0.3, 0.5&lt;w.ltoreq.1) or A.sub.2.sup.III As.sub.u P.sub.1-u (A.sub.2.sup.III represents a Group III element with In composition of not less than 0.3, 0.ltoreq.u.ltoreq.1) formed on a semiconductor crystal substrate or on an epitaxial layer grown on the semiconductor crystal substrate, it is possible to prevent the said interfacial defects, especially the crystal defects caused in the heating step, and to obtain good the crystals lattice-matched to the underlying layer. The present invention has been attained on the basis of this finding.